The invention concerns a method for determining the position of seismic equipment, especially seismic sources and seismic streamers, wherein the seismic equipment is adapted to transmit and receive acoustic signals in a seismic frequency range. The invention also concerns an application of the method.
Seismic surveys can be conducted at sea, on shore or in zones between sea and shore, e.g. in shallow bays, in swampy areas and the like. A common feature of the surveys is that a seismic signal is transmitted from a source suited to the purpose and this signal is reflected by the ground formation and proceeds to be intercepted by seismic sensors. The signals are transmitted to an appropriate receiver station, where these data are processed and stored, to be used finally for constructing structural maps of the rock formations. These maps facilitate the process of assessing the probability of the existence of oil or gas in the surveyed area.
In marine surveys where it is the seabed which has to be surveyed, a typical seismic tow will consist of one or more sources and one or more cables, also called streamers. The actual towing is performed by one or more vessels. The seismic equipment towed behind the vessels is usually submerged in the water, the sources at a depth of roughly 5 to 10 meters, and the streamers at roughly 5 to 30 meters. A streamer generally extends to a length of from a few hundred meters to several thousand meters, although a length of 3,000 meters is usual. Inside the streamer which has a diameter of approximately 6 centimeters there are located a large number of sensors, also called hydrophones. A source usually consists of several suitable sonic guns, e.g. airguns which are arranged in a row or in a group. This is also called a gun array. A gun array is normally from 10 to 30 meters in length. When airguns are used, the guns are filled with compressed air, this air being released at a given time, thereby forming the seismic pulse. This is also called a seismic shot or a shotpoint. It is this pulse which, after having been reflected, is intercepted by sensors in the seismic streamer. In a streamer of approximately 3,000 meters there can be from several hundred to over a thousand groups consisting of one or more sensors. This means that the groups are situated close to one another. The normal procedure is to record data concerning the ground formation every 25 meters, and with a density in the tow's width direction of from 50 to 200 meters. The signals received by the hydrophones are transmitted analogically or digitally to a receiver station via the streamer's system of transmission lines. In the receiver system the signals are processed in the desired manner. The processing of data is performed by means of a computer program and algorithms suited to the purpose.
One of the prerequisites for obtaining the best possible result from a seismic survey is to have a method for determining the position of the seismic equipment which can provide as correct an image of the equipment's positions as possible.
On the basis of the prior art in this area, reference can be made to several methods for positioning the seismic equipment. One known method is the use of magnetic compasses in order to determine position. Magnetic compasses are attached at regular intervals, e.g. every 400 meters, along the streamer, generally with slightly closer spacing in the first and last pads of the streamer. By noting the compass readings and processing these data a determination can be made of the streamer's orientation at the point where the compass is attached to the streamer. An estimate of the streamer position must be made for that pad of the streamer which lies between the compasses. This method has several drawbacks. The magnetic compasses attached to a streamer have been found to give some doubtful readings. They are expensive to purchase. Since they are attached to the outside of the streamer they are liable to fall off. Another problem is that the actual compasses may be wrongly calibrated, thus giving misleading readings. In bad weather with high seas the compass reading may be found to give incorrect values. Even in weather conditions with a calm sea the compasses and the streamer will be exposed to movements from the sea, and a compass reading will therefore always involve some degree of uncertainty.
Another method has been the use of acoustic transmitters and receivers, also called transponders, located on the equipment, generally both on the streamers and the sources. The acoustic transmitters transmit ultrasound, which is recorded and possibly transmitted back by corresponding transponders which are attached to the equipment. The transponders are normally attached to the foremost and hindmost pads of the streamer in order to obtain the best possible information concerning the positions of a streamer and a gun array in this area. Once again compass readings have to be relied upon for the rest of the streamer. The main reason why acoustic transponders are not attached along the rest of the streamer is that it will be too expensive. To have a great deal of equipment suspended from the streamer will cause unnecessary noise, it will naturally take more time to deploy all the equipment, and the chances of losing valuable equipment will increase. In addition complicated computer programs are required in order to process and calculate a streamer's position on the basis of a complicated set-up with information from transponders and compass.
Yet another known method is the employment of surface positioning, where use is made of satellite receivers, e.g. GPS receivers (Global Positioning System). A method of this kind will normally employ GPS receivers on the streamers' end buoys in addition to the satellite receiver on the vessel. GPS receivers may also be placed on the floats of the gun arrays. By means of radio telemetry, communication is obtained between the various GPS receivers and the system on the vessel. The weakness with such a method is, of course, that the streamers are substantially submerged in water, thus making surface positioning impossible. Even though the method will give a good result for the position of the streamers and possibly the gun arrays, where it is possible to utilize surface positioning, i.e. for the foremost part of the streamers and for the streamers' end buoys, there will only be the compass readings to depend on for the main part of the streamers, which is submerged in water. In addition it is an expensive and complex manner of acquiring information on the position of the equipment.
A combination of the above methods is perhaps that which is most commonly used today. By employing surface positioning, acoustic positioning equipment such as ultrasound-based transponders on the foremost and hindmost parts of the streamer, and finally readings of magnetic compasses attached along the streamer, the positions of the seismic equipment are determined. It is clear to see that this is a very complicated method, as well as an expensive one. The drawbacks already described will also apply to this method.
U.S. Pat. No. 5,136,613 discloses a method for acoustic communication between transponders which on a given signal transmit ultrasound signals for use in acoustic positioning. The method employs the technique of spread spectrum, but nevertheless is different from the present invention, since ultrasound signals are employed here, and it is the same type of transponders which also receive the signals. A frequency multiplex system is described, in which combinations of bit pairs (00, 01, 10, 11) are encoded as predefined ultrasound frequencies. This differs from the present invention where orthogonal spread spectrum codes are used in the seismic frequency range.
U.S. Pat. No. 4,951,263 discloses a method for localizing objects under water or on the seabed, where a spread spectrum technique is used. Equipment which has to be localized may be, e.g., unexploded torpedoes, etc. Spread spectrum coding is used in order to improve the penetration capacity of the transmitter signal and to prevent undesirable localizing of the transmitter.